Self-contained air conditioning units of the reversible type which are adapted to be mounted in the outer wall of an enclosure and utilized for heating the air from the enclosure during the winter and cooling the air from the enclosure during the summer comprises a housing divided into an indoor section and an outdoor section. An indoor heat exchanger is disposed in the indoor section while an outdoor heat exchanger and usually the compressor are located in the outdoor section. The compressor is reversibly connected to the heat exchangers so that the indoor heat exchanger functions as an evaporator when the unit is operating on the cooling cycle and the outdoor heat exchanger functions as the evaporator on the heating cycle. Suitable independent fan means are provided for circulating indoor air over the indoor heat exchanger and outdoor air over the outdoor heat exchanger during operation of the system on either the heating or cooling cycle.
Under certain operating conditions, the outdoor heat exchanger functioning as the evaporator may operate at such a low temperature as to cause the accumulation of a coating or layer of frost thereon. Since such a frost layer operates as a barrier to heat transfer between the evaporator and the air being circulated over the evaporator, the efficiency of the unit is markedly reduced. Also, unless means are provided for interrupting this accumulation of frost, the evaporator can become completely filled with a layer of frost which may eventually cause motor or other damage to the unit.
U.S. Pat. No. 4,024,722-McCarty, assigned to the General Electric Company, assignee of the present invention, discloses a heat pump control circuitry including a pair of thermostats comprising sensing elements subjected to both ambient and selected component surface temperatures whereby the operation of the heat pump is maintained as long as the temperature sensed by the thermostat is above a preselected frosting level and is interrupted by the coldest preselected temperature sensed by the elements.
U.S. Pat. No. 3,159,981-Huskey, assigned to the General Electric Company, assignee of the present invention, discloses a self-contained air conditioning unit including a reversible refrigeration system and a control circuitry designed to interrupt the operation of the refrigeration system whenever either the outdoor or indoor heat exchanger attains a frosting temperature and to supply auxiliary heat to an enclosure whenever the operation of the refrigeration system is thus interrupted during a heating cycle. The frost control switch includes a vapor-filled bellows and capillary tube sensing element connected to the bellows. The tube is arranged with a first portion in contact with one of the heat exchangers and a second portion in contact with the other heat exchanger whereby the bellows-operated switch will stop the compressor when either heat exchanger attains a frosting temperature.
Another prior art attempt at solving the frosting of the outdoor heat exchanger when in the heating cycle it is operating as an evaporator, is disclosed in U.S. Pat. No. 3,466,888-Kyle. The control circuitry includes a first thermistor in heat exchange contact with the outdoor coil and a second thermistor is exposed to the temperature of the outdoor air. The two thermistors are connected in series. When frost forms on the surface of the outdoor heat exchanger, the temperature of the refrigerant therein decreases, and the voltage at the junction of the thermistor changes, and operates a control circuit which stops the fan of the outdoor coil, and reverses the flow of refrigerant so that the outdoor coil operates as a condenser coil to melt the frost.
U.S. Pat. No. 3,348,607-Cootey discloses a split-bulb or dual-bulb thermostat wherein a sensing element is located in the path of return air and the other in the outdoor or ambient air. The sensing elements are proportioned in size relative to each other and both elements transmit motion to a single power element in response to changes in the temperature of the air effecting the elements.